Silver strike electrolyte and process of plating



Patented Oct. 10, 1950 SILVER STRIKE ELECTROLYTE AND PROCESS OF PLATING William M. Tucker, Rochester, and Robert L.

Flint, Point Pleasant, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing.

- 6 Claims. 1

This invention relates t electroplating and more particularly to the electrodeposition of silver.

Because of the nobleness of silver with respect to many metals commonly silver plated and because of the very low deposition potential of silver in the conventional cyanide bath it is necessary to employ so-called strike solutions for initiating the deposition of silver on most metals. These strike solutions usually contain an appreciably lower silver content and a very much greater free cyanide content than that conventionally employed in the standard silver plating solution. In other words the successful electroplating of a metal surface with adherent silver deposits depends upon the avoidance of silver deposition by immersion and the increasing of the deposition potential of silver. Both of these effects can be controlled by regulating the silver ion concentration and the vervoltages of silver and hydrogen in the plating bath. Heretofore the primary ion concentration as well as its relative potential or overvoltage in respect to the metal surface being plated has been controlled by altering the silver content of the solution and/or by adding free cyanide to the solution. In some cases when plating a ferrous metal a small amount of cuprous cyanide is also added to the silver strike solution.

The following plating solution compositionis suggested in prior art literature for the silver striking of steel:

Formula I Gms./1. Silver (as metal) 0.75 to Free NaCN 60. to 150 To silver plate other basis metals and as a second strike for ferrous materials, the following plating solution is discussed by contemporary electroplaters.

A typical silver-copper type strike solution preferred by some as an initial strike for ferrous metals is the following:

Formula III Gms./1. Silver (as metal) 1.2 Copper (as metal) 7.0 KCN 68.0

ApplicationSeptember 19, 1945, Serial No. 617,452

These conventional silver strike formulae demonstrate the aims of the prior art namely to depress the efficiency of silver deposition and to increase the eificiency of hydrogen deposition by reducing the silver metal content and by increasing the cyanide content. It is obvious. when the purpose of employing the strike solution is to deposit a silver coating on the article being plated the use of only 0.75 gm./1. of silver in the solution nearly defeats this purpose because of the attendant control difiiculties. Likewise using such excessive quantities of NaCN is wasteful.

An object, therefore, of the present invention is an improved silver strike solution containing q a substantially greater quantity of silverand a substantially lesser quantity of free cyanide than is employed in the silver strike solutions now gen erally employed in the silver plating industry.

Another object is to provide a more economical process for electroplating silver. Other objects will appear hereinafter.

In accordance with the invention we have found that it is much more practical to lower the efficiency of silver depositions from a silver strike bath by the addition of a ferroand/or a ierricyanide to the solution. This'permits a substantial increase in the silver content of the bath and a substantial reduction in the amount of free cyanide over that undesirably high amount now commonly found necessary in baths of this character.

This electrolyte solution may be employed in a conventional manner to plate silver on different metal articles which are made the cathode in be understood that these examples while show-' ing' preferred embodiments of our invention are included merely for purposes of illustration and not as a limitation thereof.

Example 1 An article made of stainless steel was made the cathode in an electroplating bath containing 3 grams per liter of free silver, 50 grams per liter of sodium cyanide, grams per liter sodium carbonate and 25 grams per liter potassium ferrocyanid'e. An. electric currentof 0.05 ampere per square inch of cathode area was passed 7 3 through the solution. By actual measurement the efl'lciency of the silver deposition was found to be 26%. It was found that at this efficiency an improved plate resulted of excellent quality and adherence.

Example 2 Another article made of stainless steel was made the cathode in an electroplating bath containing the same amounts of bath constituents as in Example 1 with the exception that the amount of potassium ferrocyanide was increased to 50 grams per liter. The plating was conducted at a current density of 0.05 ampere per square inch employing a silver anode. The measured efliciency of silver deposition was 21.0%. The plate exhibited excellent adherence.

Example 3 Another article of carbon steel was made the cathode in an electrolyte containing 3 grams per liter of free silver, 50 grains per liter of sodium cyanide, 60 grams per liter of sodium carbonate, 50 grams per liter of potassium ferrocyanide and 25 grams per liter of potassium ferricyanide. Again a current density of 0.05 ampere per square inch of cathode area was employed. An excellent tenaciously adhering silver plate was ob tained in the cathode.

While we prefer to employ potassium ferroand ferricyanides in our plating electrolytes, we may employ other cyanides such as sodium ferroand ierricyanides.

The following table further illustrates the influence of potassium ferroand ferricyanide on the strike solution:

ponent amounts and current densities, we have found that the best results have been attained within the range of 20 to 30% efficiency in silver deposition. Particularly good results were obtained at 25% efiiciency in silver deposition and at 0.05 to 0.075 ampere per square inch of cathode area. In such case the bath contains 15 to grams of ferrocyanide per liter or 15 to 30 grams of ferricyanide per liter.

Stainless steel containing 18% chromium, 8% nickel, with a maximum carbon content of 0.08% was struck in solutions identified in the table as Nos. '7, 8, and 10 and exceptionally good adherence of the silver plate resulted. The adherence of the silver strike to the stainless steel was sufilcient to eliminate the need for both a supplementary strike'such as in a nickel chloride-H01 solution prior to silver striking as well as any further strike before final plating.

Various carbon steels similarly displayed excellent adherence when struck with these solutions.

Subsequent to the silver striking operation which is carried out in apparatus well known by those skilled in the art, the article can be transferred in accordance with usual practices to a standard silver plating bath and there receive a final relatively thick silver plate.

The efficiencies of the silver strike solutions were determined by comparing the weights of Silver deposited in a known time, with the weights of copper deposited in a copper coulometer connected in series with the silver cell. The copper solution used in the coulometer was the follow- Grams/l. CuSO4-5H2O 150 H2SO4 s- C2H5OI-I -Q. 50

The cathode current density was always less than 0.065 ampere per square inch so as to guarantee essentially 100% efiiciency of copper deposition.

It will be understood that sodium carbonate is employed in the electrolyte mainly as a conductor of current, I

.. As thus described we have found that the in- 3. 5 46 Present 0. 05 82.0

0.7 15 Present 0.05 27.3

It is apparent from this table that in the usable current density range the ferroand ferricyanide content of the bath function to reduce the efficiency of the silver deposition. The quality and adherence of these plates are excellent. Flash strikes at much higher current densities also demonstratea similar efiect on the efiiciency of silver deposition from silver strike solutions containing ferroand ferricyanide. It will be understood from the above table that a range of 45 to 50 grams of sodium cyanide can be advantageously employed in the strike solution.

While, as shown in the table, the ferroand ferricyanide containing silver strike solutions can satisfactorily be employed with variations in comcorporation in silver strike solution, containing silver and free cyanide, of sulficient ferroand/or ferricyanide to reduce the eificiency of silver deposition to between 20 to 30% at current densities under 0.1 ampere per square inch of cathode area grams of a material selected from the group consisting of an alkali ferrocyanide, an alkali ferricyanide and mixtures thereof.

2. A silver strike electrolyte, adapted to re duce the efiiciency of the silver deposition on a cathode to a range of to 30% consisting, in water solution, of 3 grams of silver per liter, 60 grams of sodium carbonate per liter, 50 grams of sodium cyanide per liter and grams of potassium ferrocyanide per liter.

3. A silver strike electrolyte, adapted to reduce the efilciency of the silver deposition on a cathode to a range of 20 to consisting, in water solution, of 3 grams of silver per liter, 50 grams of sodium cyanide per liter, 60 grams of sodium carbonate per liter, 50 grams of potassium ferrocyanide per liter and 25 grams of potassium ferricyanide per liter.

4. A silver striking process which comprises installing the metal article to be plated as the cathode in an electrolyte provided with an anode, said electrolyte consisting, in aqueous solution, of 2 to 3.5 grams of silver per liter, to grams of sodium cyanide per liter, grams sodium carbonate per liter, and 15 to 50 grams of a material selected from the group consisting of an alkali ferrocyanide, an alkali ferricyanide and mixtures thereof, and electrolyzing the electrolyte by employing a current density of less than 0.10 ampere per square inch of cathode area whereby a plating efficiency of silver on the cathode within the range of 20-30% is achieved.

5. A silver striking process which comprises installing the metal article to be plated as the cathode in an electrolyte provided with an anode, said electrolyte consisting in aqueous solution of 3 grams or silver per liter, 50 grams sodium cyanide per liter, 60 grams sodium carbonate per liter, and 25 grams potassium ferrocyanide per liter, and electrolyzing the electrolyte by employing a current density of 0.05 ampere per square inch of cathode area, whereby a plating efiiciency of silver on the cathode within the range of 20-30 is achieved.

6. A silver striking process which comprises installing the metal article to be plated as the cathode in an electrolyte provided With an anode, said electrolyte consisting in aqueous solution of 3 grams of silver per liter, 50 grams sodium cyanide per liter, 60 grams sodium carbonate per liter, and 50 grams potassium ferrocyanide per liter, and electrolyzing the electrolyte by employing a current density of 0.05 ampere per square inch of cathode area, whereby a plating efiiciency of silver on the cathode within the range of 20-30% is achieved.

WILLIAM M. TUCKER. ROBERT L. FLINT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 58,037 Thompson Sept. 11, 1866 643,096 Christy Feb. 6, 1900 OTHER REFERENCES Transactions of the Electrochemical Society, vol. 23 (1913), pages 84 and 85; vol. 24 (1913), pages 241-270; vol. 76 (1939), pages 383-387; Preprint 74-30, Oct. 17, 1938, pages 453-467. 

1. A SILVER STRIKE ELECTROLYTE, ADAPTED TO REDUCE THE EFFICIENCY OF THE SILVER DEPOSITION ON A CATHODE TO A RANGE OF 20 TO 30% CONSISTING, IN WATER SOLUTION, OF 2 TO 3.5 GRAMS OF SILVER PER LITER, 45 TO 50 GRAMS OF SODIUM CYANIDE PER LITER, 60 GRAMS SODIUM CARBONATE PER LITER, AND 15 TO 50 GRAMS OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF AN ALKALI FERROCYANIDE, AN ALKALI FERRICYANIDE AND MIXTURES THEREOF. 